The present disclosure generally relates to magnetic composites and more specifically to magnetic pastes for use in high frequency applications.
The integration of magnetic components into electronic circuits has become a significant barrier to reducing the size of electronic devices. Current micrometer sized magnetic materials can only be used at low frequencies. For example, high permeability bulk ferrites can only be used at frequencies less than 1 megahertz (MHz) and bulk Ni-ferrites, which may be used up to about 100 MHz, have non-optimal initial permeabilities. Accordingly, existing commercial converters are designed to operate at frequencies less than about 2 MHz.
One method for integrating higher frequency magnetic devices into a circuit is through thick film screen-printing. Thick film screen-printing allows for the deposition of a magnetic material in paste form onto a ceramic substrate. The paste material generally consists of ceramic or metallic particles suspended in a polymer paste. Unfortunately, the permeabilities of these pastes are very low (e.g., less than about 3), resulting in large eddy current losses.
After printing, the paste undergoes a heat treatment that ultimately burns off the polymer and densifies, or sinters, the ceramic or metallic material. The result is a dense pattern formed from the ceramic or metallic material. Using this technique, for example, a conducting coil can be printed onto a layer of ferrite, and then covered with another layer of ferrite to form an inductive coil. Successive layers can be printed to increase the inductance of the device. The heat treatments, result in increased permeabilities, but are accompanied by increased eddy current losses at higher frequencies. Furthermore, the high temperature (about 700 to about 900 degrees Celsius) of the heat treatment makes these pastes incompatible with low temperature printed wiring board (PWB) or printed circuit board (PCB) processing procedures.
Accordingly, there remains a need in the art for new and improved magnetic pastes that have a desirable combination of properties, such as inductance, permeability, and/or permittivity at the desired frequency, and that may be used in devices that are processed at low temperatures.